1. Field of the Invention
This application relates to communication cables. More particularly, this application relates to loose-tube type fiber optic cables.
2. Description of the Related Art
In the area of fiber optic cables, there are many different designs, each of which has some purpose both in fiber count, mechanical properties, environmental resistance properties, fire resistance/smoke, etc. . . . Among the various designs, mid count-designs (i.e. more than 12—less than 200 fibers) typically contain the fibers in a loose tube style arrangement. “Loose tube” is a commonly understood term designating a fiber cable design that has a jacket, at least one buffer tube inside the jacket with at least one (usually more) UV coated optical fiber(s) loosely contained inside each buffer tube.
More particularly, the “loose” term in “loose tube” refers to the fibers being loose within buffer tube thus allowing the fibers to reside within a relatively free space. Within this free space the fibers have the ability to bend/move (such as into a sinusoidal shape or minimally helical shape) along the length of the cable, accumulating as the cable (jacket and tubes) contracts over cold temperature extremes. By allowing for this “loose” room within the buffer tubes, the fibers are able to avoid the stresses imparted by the cold temperatures on the tubes and jacket and thus likewise avoid undue attenuation.
For example, within the area of loose tube type optical fiber cables, the designation “LT” is a standard for a tube approximately 3 mm in outside diameter (herein after “OD”) and 2 mm inside diameter (hereinafter “ID”) with a corresponding 0.5 mm wall thickness. Some variations on these dimensions are also available in the prior art. The interior fibers have ample free space to move.
On the other hand, a different type of fiber optic cable is the so-called Tight Buffer or “TB” type cable were where the optical fiber is tightly encapsulated with plastic. These encapsulated fibers are subsequently grouped in subunits.
Focusing on loose tube type arrangements, FIGS. 1 and 2 show some basic prior art designs for mid-count loose tube type fiber optic cables FIG. 1 shows a first prior art loose tube fiber optic cable for one hundred and forty four (144) fibers bundled into twelve independent buffer tubes, twelve fibers each, arranged around a strength member. FIG. 2 shows a separate prior art arrangement for a loose tube fiber optic cable for one hundred and forty four (144) fibers bundled into thirty six independent buffer tubes, four fibers each, divided into three twelve tube subunits inside the larger outer jacket.
Some prior art designs such as those shown in FIG. 2, akin to the loose tube LT design, employ smaller and thinner buffer tubes to protect the fibers. For example, in some cases, the buffer tubes may have very thin walls and have very low interior space and can be referred to as thin wall low interior space tubes (or “TWLIS”) or “TL” for short (T=Thin Walled and L=Low Interior Space). Such TL tubes, apart from protecting the fibers when the jacket is removed (e.g. for connectorization), are also used to help bundle and identify the various fibers within (e.g. by color grouping).
However, even though such prior art designs are adequate for basic protections of the fibers and organization purposes they have problems particularly when trying to address the mechanical and smoke/fire properties of buffer tubes and jackets within such TL type loose tube cables.
There has been many variations of TL tubes in the market but they tend to be unreliable when four (4), six (6) and twelve (12) fibers are enclosed within.
For example, one prior art type cable provides an equation where the desirable Young's modulus for the buffer tube material should be:                Y=590×(tube wall thickness)−1.2         (wall thickness is in mm)        (material modulus is in PSI)        
Using the examples in the prior art with a nominal range of buffer tube wall thickness between 0.2 and 0.3 mm (est. 0.25 mm) the equation shows the calculated ideal modulus to be590*(0.25)−1.2=3114 psi
This is relatively close to the prior art disclosure of using polymers for buffer tubes having a 2500 psi modulus.
However such modulus measurements change relative to the point at which they are measured. For example the modulus of plastic changes during the extension of the plastic during extrusion processing, handling (as discussed in more detail below with respect to the tensile test tables). Likewise, the modulus of a plastic at the beginning of a tearing process would be much greater than at the end of the tearing process (tearing refers to the activity of pulling on a tube until it tears with the plastic yielding and fracturing for the purpose of opening to gain access to the fibers).
Another feature of fiber optic cable construction is the buffer tube wall thickness. Some tube wall thickness ranges recommended by the prior art are in the range of 0.2 mm to 0.3 mm (0.00787″ to 0.0118″) or roughly half what standard loose tube buffer tube wall thickness had been in the past 20 years (0.5 mm (0.0197″)). Such prior art designs devote emphasis to 0.2 mm to 0.3 mm tube wall thickness ranges based on the ability to easily tear such tubes open for access to the fiber therein, or to have the strength in a semi-tube or patch cord construction providing protection when connectorized.
However, there are many other cable design features, such as overall cable diameter per number of fibers, mechanical, signal quality and fire/smoke standards, each that need to be addressed. One important aspect is the ability of the fibers to resist the contraction forces in such tight constraints or where the anti-buckling resistance of the fibers must match the contraction strength of the tube, at for example −60° C.
Likewise, fiber optic cable designs generally need to meet basic attenuation standards which are typically 3.0/1.0 dB per km at wavelengths of 850 and 1300 nm for multimode fiber and 0.4/0.3 dB per km at wavelengths of 1310 and 1500 nm for single mode fiber, with some related attenuation standards allow changes under shrinkage temperature standards which are typically set at less than 0.30 dB per km.
In addition to attenuation and attenuation under cold temperatures, other related mechanical testing issues may include but are not limited to—                Hot and Cold Bend (e.g. FOTP (FOTP=TIA Fiber Optic Test Procedure) 37A);        Room, and Cold Impact Resistance (e.g. FOTP-25C);        Compressive Strength (e.g. FOTP-41A);        Tensile Loading and Bending (e.g. FOTP-33A);        Twist (e.g. FOTP-85-A);        Cable Flexing (e.g. FOTP-104A);        Jacket Tensile Strength and Elongation (e.g. FOTP-89A);        Jacket Shrinkage (e.g. FOTP-86A);        Temperature Cycling (e.g. FOTP-3);        Cable Aging and Color Permanence (e.g. FOTP-3).        
In each case, although the prior art suggests using thinner walled buffer tubes in their TL designs for certain access and connectorization issues, the thinner walls and lower amounts of free space for the fibers therein makes these tests more difficult to pass.
In addition to the above discussed mechanical and signal considerations, fiber optic cables also need to meet fire/smoke standards which may include the NFPA 262 Plenum Test also known by a prior name of UL 910 Test, where cables are laid side by side in an approximately 11¼″ width and a 25 ft length, and subjected to flame and air flow whereby the measured flame length (red char) must be under a limit of 5 ft and where the smoke emitted must pass between an emitter and a detector with the calibrated peak limit of 0.5 (optical density—light intensity pass-through percentage reduction) and have an average limit for the duration of the test of less than 0.15.
One of the key fuels within the cable in such fire tests that produces smoke is the UV coating on the optical fibers. Prior art thin buffer tube wall designs (e.g. 0.20 mm-0.30 mm) make passing these types of fire/smoke tests more difficult than the normal wall thickness (0.50 mm) due to the faster exposure of the of the higher density UV fiber coatings (fuel) to the flame, and the higher density of the fiber coating within a given cable diameter.
Yet another issue with thinner walled buffer tubes is that the extrusion process for the TL sheaths is difficult to process because the excess fiber length must be controlled more precisely than in the larger buffer tubes having more space. When buffer tubes are extruded onto fibers, the fibers are provided with an excess length to give the fibers some range of movement within the tubes during bending, cold/warm temperature shrinkage/expansion, etc. . . . However, with thin walled buffer tubes with less open space for the fibers, there is much less space in which to accumulate the variability of excess fiber length, so the excess lengths of the fibers must be more tightly controlled.